1. Field of the Invention
The present invention is directed to a method for sealing membranes, especially heat sealable thermoplastic membranes and, more particularly, to such a method whereby the seal between said membranes, especially heat sealable, thermoplastic membranes is effected by a hot air welding technique.
2. Description of the Prior Art
A recently emerging trend in the field of roofing is the displacement of the familiar built-up asphalt roofing with membrane roofing. Advantages of the new systems include ease of installation, a safer working environment for the applicator, greater resistance to cold cracking and longer term leak protection.
Another leading growth area is the utilization of ponds and pits for disposal of wastes of chemical plants, petroleum refineries, power plants and mining installations. To insure against seepage of aqueous wastes, liners must be provided. Thus, the development of pond and pit liners is a growing industrial concern.
Two membrane types, elastomeric and thermoplastic, are primarily utilized in this new technology. A leading elastomeric membrane is vulcanized EPDM while examples of thermoplastic membranes are plasticized PVC, chlorinated polyethylene (CPE) and chlorosulfonated polyethylene (CSPE).
Both membrane types have advantages and disadvantages. Vulcanized EPDM has outstanding resistance to outdoor weathering, good cold flexibility, high strength and excellent elongation characteristics. Its major disadvantage is the necessity of using adhesives for seaming the membrane to provide a continuous leak-free roof covering. Such adhesives are expensive and time-consuming to apply and are prone to delaminate under stressful conditions because of their low strength resulting in leakage at the delaminated sites. Plasticized PVC offers surer seams because the material, being thermoplastic, can either be heat sealed or solvent welded to give an integral seam of high strength. Such membranes, however, tend to lose plasticizer with time resulting in a short useful life and poor cold crack resistance.
In recent years, new classes of polymers have been developed which are based on chemically modified elastomers, especially saturated backbone elastomers such as EPDM and Butyl rubber. More specifically, the polymers are modified in such a way as to introduce an ionic group on the polymer. Thus, they may be referred to, in general, as "ionomers" or "ionic elastomers".
Among this new class of "ionomers" are sulfonated elastomers and carboxylated elastomers. These polymers are derived from polymeric materials having olefinic unsaturation, especially elastomeric polymers such as EPDM rubber. U.S. Pat. No. 3,642,728, incorporated herein by reference, teaches a method of selective sulfonation of olefinic unsaturation sites of an elastomeric polymer to form an acid form of a sulfonated elastomeric polymer. The olefinic sites of the elastomeric polymer are sulfonated by means of a complex of sulfur trioxide donor and a Lewis base. The SO.sub.3 H groups of the sulfonated elastomer are readily neutralized with a basic material to form an ionically crosslinked elastomer having substantially improved physical properties over an unsulfonated elastomer at room temperature. However, these ionically crosslinked elastomers may be processed like a conventional thermoplastic at elevated temperatures under a shear force in the presence of selected preferential plasticizers which dissipate the ionic associations at the elevated temperatures thereby creating a reprocessable elastomer.
Further developments relating to "ionomers", i.e., the sulfonated polymers, are disclosed in U.S. Pat. Nos. 3,836,511; 4,222,914; 3,870,841; 3,847,854 and 4,157,992.
A second new class of elastomeric "ionomers" is the class of carboxylated elastomers. These polymers are derived from polymeric materials having olefinic unsaturation, especially elastomeric polymers such as EPDM. See, for example, U.S. Pat. No. 3,884,882 and 3,997,487, both of which are incorporated herein by reference.
Although these ionomers represent a significant development in the elastomeric arts, none of these advances have found application in the waterproof membrane arts. That is, none of the aforementioned disclosures explicitly suggest, implicitly imply, or otherwise make obvious their use in any application which requires a waterproof membrane, covering or the like wherein the membrane or covering requires long term exposure to the elements.
More recently, commonly assigned U.S. Pat. No. 4 480 062, incorporated herein by reference, discloses such a waterproof membrane comprising an elastomeric composition. Commonly assigned U.S. Pat. No. 4,589,804 incorporated herein by reference, also discloses the use of such a composition to waterproof a surface and the resultant roof formed thereby.
In general, such a waterproof membrane comprises an elastomeric sheet which includes a neutralized acid group containing elastomeric polymer, the neutralized acid group containing a cation selected from the group consisting of ammonium, antimony, aluminum, iron, lead and a metal of Groups IA, IIA, IB, or IIB of the Periodic Table of Elements and mixtures thereof. The sheet may also include a non-polar process oil, carbon black and a preferential plasticizer.
Examples of preferential plasticizers are selected from the group consisting of a basic salt of carboxylic acid having 2 to 30 carbon atoms. The salt contains a cation selected from the group consisting of antimony, aluminum, iron, lead, a metal of Group IA, IIA, IB, or IIB of the Periodic Table of Elements and mixtures thereof and an organic amide having the formula R.sup.1 CONR.sup.2 R.sup.3 where R.sup.1 is an aliphatic group and R.sup.2 and R.sup.3 are the same or different and are hydrogen, alkyl, aryl, aralkyl or the group -CH.sub.2 CH.sub.2 NHCOR.sup.1, and where at least one of R.sup.1, R.sup.2 and R.sup.3 has at least 5 carbon atoms.
Elastomeric polymers which can be used in the composition include EPDM, Butyl rubber, thermoplastic polyolefins and the like.
Other additives can be added to the composition such as antioxidants, fillers and the like. Other nonessential features of the elastomeric composition can be obtained from U.S. Pat. No. 4,589,804.
The foregoing references are illustrative of the different types of compositions which can or cannot be calendered or extruded to form waterproof coverings or membranes.
The present invention relates to a new method for sealing membranes that are heat-activatably bondable, particularly for heat sealing heat sealable materials by hot air welding, whether they be in the form of sheets, membranes, film, etc. Thus, in a broad sense the term "heat seal" means the process wherein at least a portion of two membranes, at least one of which has a heat-activatably bondable surface, are permanently affixed together either by the use of hot air alone which causes the overlapped surface of at least one of the two membranes to soften and fuse i.e., self-seal or by the use of hot air impinging upon heat-activated adhesive means positioned between said portion of two membranes to soften and render effective said adhesive means. By the term "heat activatably bondable" where applied to a surface is meant a surface that is thermoplastic and therefore capable of fusing at elevated temperatures or one that has some adhesive means affixed thereto that is rendered effective or activated by elevated temperatures. The term "membrane" as used herein is meant to include films and/or sheets which are capable of being heat sealed.
One of the principal applications of the above discussed membranes is as a roof covering. The types of roof which may be covered by such membranes are flat or sloped roofs and may comprise new or retrofitted roof installations. The type of roof surface which may be covered by such membranes, often referred to as the roof deck, may be wood, cement, metal, concrete or combinations thereof. In addition, such membranes may also be affixed to insulation which may be disposed over the aforementioned roof decks. Insulation such as wood fiberboard, expanded polystyrene, fiberglass board and rigid polyurethane board may be covered with the unreinforced membranes of this invention. In installing the roof covering, the covering may be fully bonded to the roof surface, spot or partially bonded to the roof surface, loose laid and ballasted, or mechanically bonded by methods such as battens, discs or buttons.
In addition, such membranes may also be employed as roof flashing, cover roof protrusions, drains, gutters, outlets, edge trims, parapet wall terminations, corners and other roof details.
In general, the aforementioned membranes can be seamed together by various techniques, including heat sealing, solvent welding, adhesive bonding and elastomeric adhesive tape bonding, or combinations thereof. The method of the present invention is, however, particularly effective and therefore especially directed to a hot air welding technique for heat sealing. In this technique, the membranes or panels are normally "lap-seamed" to create a "lap-seam assembly" which provides the large required panel sizes necessary for roof coverings, pond liners and the like.
In a preferred embodiment of this invention, the actual lap-seaming of the membranes is effected by hot air welding which provides a high strength integral bond without the introduction of any foreign material. This is accomplished by providing a lap-seam assembly where the membranes meet and directing a hot air flow between the laps from a hot air gun followed by a pressure roller along the top of the seam. Unfortunately, however, when utilizing such a hot air gun roller, various problems can arise when attempting to seal the panels together. When the top sheet is heated to fusion temperatures, the top sheet softens and thermally expands in front of the hot air gun roller. This expansion is particularly noticeable with unreinforced membranes. This expansion causes the membrane material to build up in advance of the roller, until it finally folds or distorts and passes beneath the pressure roller wheels. This produces an aesthetically unpleasing finished product which may have a fold every few feet on the seam. More significantly, such folds serve as a possible source of leakage.
It is therefore an object of the present invention to provide a method of joining together unreinforced sheets or membranes, especially heat sealable, thermoplastic sheets or membranes.
It is a further object of the present invention to provide a method for sealing together lapped edge portions of sheets or membranes without distorting the edge portions of the membranes.
It is still a further object of the present invention to provide a method of sealing unreinforced membranes by the hot air technique especially of heat sealing by the hot air welding technique, heat sealable, unreinforced thermoplastic sheets or membranes such that the resultant seal is devoid of any wrinkles and/or billowing effects.
It is yet a further object of the present invention to provide such a method of sealing with or without adhesive means, such that the resultant seal created maintains its waterproof qualities over extended periods of time.
These and other objects of the present invention will become apparent to those skilled in the art.